Ok, I can put a custom button inside the doc
and use a CloseCommand(DelegateCommand) for closing the doc like below.
But I think, using the already existing DocumentClosButton instead of
adding an additional one would be better.

Thanks for your feedback. It seems your question is about XamDockManager and not directly related to NCAL, so please post the question on Infragistics'
XamDockManager Forum. That way, your question and answer will be easily discoverable by other developers. Thanks!